Same has happened to me just after Testosterone has fully returned. My concern was it might be an indication of more breast growth (which is generally what the pain is). I don't understand what causes this effect at this time. I have asked an Andrologist friend but he hasn't got back to me yet. I still have Tamoxifen on prescription "dose according to symptoms", although not needed it for a while. Put myself back on the Tamoxifen, and it got rid of the pain in a day.
If your situation is the same, I suspect it's happened when Testosterone has returned (timeframe is about right), and I have had a few guys mention this in the past when coming off hormone therapy. I don't think this is lack of Testosterone - mine was almost 20nmol/l two months ago, although I'd need another test to be sure it hadn't dropped for some reason.